Condensation in Buildings - Australian Building Codes Board

Information Handbook: Condensation in Buildings
selected for the air control function and its installation would need to produce an airtight surface
but its resistance to water vapour transmission would depend on the vapour control strategy
developed in response to the issues discussed in Section 5.7.
This removes practical obstacles to installing an effective vapour control layer on the interior
side of insulation but a high resistance material would be appropriate only in cold climates (and
may not be necessary, even there). This is not a suitable location for a highly impermeable
material in a tropical climate or in a climate which will encourage air conditioning use in
summer. Depending on the permeance of other layers closer to the exterior, extra water vapour
resistance might be beneficial in cool temperate climates.
5.7 Water Vapour Control Layers
Once it is clear that the dew point temperature of either the indoor or outdoor atmosphere is
substantially higher than likely interstitial surface temperatures, there will be an understandable
impulse to put a barrier between the water vapour and those colder surfaces to fix possible
condensation problems once and for all. A disappointing complication of that approach is that
North American and European experience in recent decades has shown that the barrier might
solve the problem for one season but not for all.
In very cold climates, vapour barriers installed on the warm side of the envelope aim to limit the
migration of warm indoor water vapour through building fabric which becomes progressively
colder in its outer layers. In tropical climates, the same logic allows a vapour barrier on the
warm, outdoor side of the envelope to restrain water vapour driving towards surfaces cooled by
an air conditioned interior. This approach can work in persistently cold climates. It can work in
constantly warm and humid tropical climates. When used in climates with mixed seasons, it has
been found to create more difficulties than it solves.
Cold climate vapour barriers were extensively adopted in warmer climates across the United
States during the 1990s as the answer to winter season condensation. The polyethylene
membranes installed as vapour barriers behind interior linings were soon revealed as sites of
condensation during summertime air conditioning. Reflecting on this experience, in Errata to his
Moisture Control Handbook, in June 2002, Joseph Lstiburek observed:
“In the past decade several things have become obvious to me that are not reflected
adequately in the Handbook. The most significant is the role of polyethylene on
retarding the drying of building assemblies. I have come to conclude that
polyethylene is really a ‘drying retarder’ and should be avoided. Polyethylene should
not be installed on the interior of any assembly – with the exception of above grade
walls and ceilings in locations with 8,000 heating degree days or greater.”
DRAFT HANDBOOK
Only the alpine climates in Australia are likely to meet this test. Although some locations in
Australia have just one dominating season, most have mixed climates. The growing use of air
conditioning in dwellings (with 0.7 air conditioners per household when assessed in 2006)
Australian Building Codes Board Page 67